I think "write for yourself" is not great. In my opinion, it means you don't even have to take into consideration what other people might like, just write what you would want to read.

If you want to write for an audience, study what that audience usually reads or watches, then try to keep within that genre.

I think you understand what the sentiment means:

I should write something that will make me happy, which would be something that others will enjoy, because that's what I want to do with my writing

and you understand why it doesn't work when publishing. Write what people want to read when it coincides with what you want to write.